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James Watson and Francis Crick discovered the structure of DNA in 1953. The shape of DNA is often likened to that of a twisted ladder, or a double helix. The alternating sugar-phosphate groups make up the sides of the ladder. The rungs of the ladders consist of two bases held together by hydrogen bonds that separate easily during replication.
One half of the DNA ladder, or polynucleotide chain is often
labelled with one end called the 3' "Three Prime" end and the
other end called the 5' "Five Prime" end as shown. They are
distinguished as follows: the 5' end begins with a phosphate group; the 3'
end begins with a hydroxide (OH-) group.
The chains of the DNA molecule run anti-parallel to each other, i.e.
5' --> 3'
3' <-- 5'
The most important feature of DNA is that one chain is complimentary to the other. The bases are paired in a definite pattern, so one base chain determines the other side, and vice versa. Thus for a sequence (5')CATTAG(3'), the complementary sequence must be (3')GTAATC(5'). This is vital for protein synthesis (specifically, transcription) and also DNA replication.
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